Large Actuation Strain Research Articles

Development of dielectric elastomeric actuators for morphing wings

In the last decades, wing morphing structures have aroused great interest due to their capability to improve the aerodynamic efficiency of modern aircraft. DE actuators, also known as “artificial muscles” due to their ability to exhibit large actuation strains at high voltages, are suitable candidates for morphing applications. This paper focuses on the research and development of miniature dielectric elastomeric actuators for variable-thickness morphing wings. A conical elastomeric actuation configuration has been proposed, consisting of a VHB4910 dielectric membrane preloaded with a spring mechanism and constrained to a rigid circular ring. The mini-actuators are developed to be fixed in an actuation array, mounted to the wing skin. This new electromechanical actuation system is designed to be integrated on thin airfoil wings, where conventional morphing structures cannot be used, because of restricted mass and space requirements. By controlling the thickness distribution using the proposed actuators, we may be able to maintain and delay the location of the laminar-turbulent transit towards the trailing edge, promoting laminar flow over the wing surface. Experimental models and prototypes will be developed in the next phase of the research project for further investigations.

Multi-Layers Planar Dielectric Elastomer Actuator Toward Reducing Control Voltage in In-Plane Actuation Applications

Dielectric elastomer actuators (DEAs) belong to an emerging soft actuation technology that have advantages in large actuation strain. However, the existing single-layer planar DE actuators commonly need fairly high voltage to achieve good in-plane actuation capability. A multi-layers planar dielectric elastomer actuator (MPDEA) is proposed in this work to realize effective in-plane actuation under low voltage. To facilitate the studies on the effects of multi-layers planar configuration of actuator on control voltage, a base flexible thin-walled beam equipped with a MPDEA is investigated. The electromechanically coupling finite element model of composite beam structure is first established. The theoretical model of actuating force of MPDEA is obtained. The numerical and experimental studies on the in-plane actuation capabilities of MPDEAs with different layer number are then carried out through evaluating the electro-driven deformation of base flexible thin-walled beam. The results demonstrate that increasing the number of layers can improve significantly the in-plane driving capability of MPDEA. As a result, a four-layers MPDEA can reduce control voltage by 53.62% compared with a single-layer planar DEA to generate the same deformation at the tip of beam.

Enabling large actuated strain at low electric field by grafting cyanoester dipoles onto a poly(styrene-b-butadiene-b-styrene) elastomer using thiol-ene click chemistry

A dielectric elastomer can undergo a large deformation when it is subjected to an applied electric field. As a result, it finds potential usage in artificial muscles, soft robotics, and medical devices. However, current dielectric elastomers suffer from low dielectric constants (e.g., <4.5). Grafting highly dipolar groups onto a dielectric elastomer has been proved to be an efficient method to improve the dielectric constant and thus the electroactuation property. However, grafting a side chain bearing two or more dipolar groups onto the polymer main chain has not been explored in the past. In this work, we grafted cyanoester groups onto a poly (styrene-b-butadiene-b-styrene) (SBS) elastomer to improve its dielectric constant and actuation performance using the thiol-ene click reaction. A series of new dielectric elastomers with different grafting densities were obtained with excellent actuation properties: a dielectric constant up to 11.6 at 105 Hz and a large actuation strain of 9% at a low field of 13.9 MV/m. This work provides us a new and effective strategy to improve the performance of dielectric elastomers.

On the Mechanical Power Output Comparisons of Cone Dielectric Elastomer Actuators

The emerging demand for bioinspired soft robotics requires novel soft actuators whose performance exceeds conventional rigid ones. Dielectric elastomer actuators (DEAs) are a promising soft actuation technology with large actuation strain and fast response. Cone DEAs are one of the most widely adopted DEA configurations for their compact structure and large force&#x002F;stroke output with several configuration variations developed in recent years. By driving at a resonant frequency, the cone DEAs show a significant amplification in their power outputs, which demonstrates their suitability for highly dynamic robotic applications. However, it is still unclear how the payload conditions could affect the power outputs of cone DEAs and no work has compared the output performance of different variations of cone configurations. In this article, by considering conical configuration DEAs with generalized dissipative payloads, we conduct an extensive study on the effects of payload conditions on the power outputs of the cone DEA family. Additionally, we benchmark the performance of different cone DEA configurations and illustrate the fundamental principles behind these output patterns. The findings reported in this article establish guidelines for designing high-performance cone DEA actuators.

Stable and High‐Strain Dielectric Elastomer Actuators Based on a Carbon Nanotube‐Polymer Bilayer Electrode

AbstractDielectric elastomer actuators (DEAs) have shown promises in numerous applications such as bio‐inspired robotics, tactile displays, tunable optics, and microfluidics, owing to their unique combination of large actuation strain, high energy density, and light weight. However, the practical applications of the DEAs have been hindered partly due to their poor reliability and durability under high‐strain actuation. A major failure mechanism is from the localized electrical breakdown. Compliant electrodes with self‐clearing capability have been studied to prevent premature failures. Here, an interpenetrating bilayer compliant electrode comprising a thin layer of a water‐based polyurethane (WPU) overcoated on an ultrathin single‐walled carbon nanotube (SWNT) layer is reported. The thin polyurethane layer serves as the dielectric barrier to suppress corona discharges of the nanotubes in air. The SWNT+WPU bilayer electrode has the capability to self‐clear at the breakdown sites, enhancing the fault tolerance and mendability of the DEA at a large‐strain actuation. Stable actuation at 150% area strain for 1000 cycles under square‐wave voltage and 5.5‐h continuous actuation at a constant voltage have been achieved for acrylic elastomer‐based DEAs.

Repeatable and Reprogrammable Shape Morphing from Photoresponsive Gold Nanorod/Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are of interest for applications such as soft robotics and shape-morphing devices. Among the different actuation mechanisms, light offers advantages such as spatial and local control of actuation via the photothermal effect. However, the unwanted aggregation of the light-absorbing nanoparticles in the LCE matrix will limit the photothermal response speed, actuation performance, and repeatability. Herein, a near-infrared-responsive LCE composite consisting of up to 0.20 wt% poly(ethylene glycol)-modified gold nanorods (AuNRs) without apparent aggregation is demonstrated. The high Young's modulus, 20.3MPa, and excellent photothermal performance render repeated and fast actuation of the films (actuation within 5 s and recovery in 2 s) when exposed to 800nm light at an average output power of ≈1.0 W cm-2 , while maintaining a large actuation strain (56%). Further, it is shown that the same sheet of AuNR/LCE film (100µm thick) can be morphed into different shapes simply by varying the motifs of the photomasks.

Morphotropic phase boundary-like effect in hybrid electrically poled, mechanically depolarized ferroelectric ceramics

The morphotropic phase boundary (MPB) plays an important role in ferroelectric materials. Typically, two phases coexist in materials near the MPB. Such materials usually exhibit large piezoelectricities, dielectricities, and actuation strains. In this work, we produce an MPB-like effect in hybrid electrically poled, mechanically depolarized (HEPMD) BaTiO3 and lead zirconate titanate ceramics where depolarized region A and vertical electrically poled region C intersect each other with a period of 400 μm in both in-plane directions. The polarization and strain of both HEPMD samples are over 3 times those of conventionally poled samples under unipolar electric loading. The large polarization and strain decrease steadily as the frequency increases and stabilize at approximately twice the values from the conventionally poled samples. Furthermore, the large polarizations and actuation strains of the HEPMD samples are fairly stable and change little after 30 000 cycles of operation. Under bipolar electric loading, the tendencies are similar and the coercive fields of the HEPMD samples are considerably smaller, which is similar to the MPB effect in traditional ferroelectric ceramics. Enhanced polarization and strain occur in HEPMD samples due to reversible ferroelectric domain switching during loading and unloading under large electric fields. In comparison, the small-signal properties, i.e., the d33 and dielectric properties, are slightly larger in HEPMD samples than in conventionally poled ones. The HEPMD method may be applied to all types of multiaxial ferroelectric ceramics to enhance actuation strain and polarization.

A phase-field approach to studying the temperature-dependent ferroelectric response of bulk polycrystalline PZT

Ferroelectric ceramics are of interest for engineering applications because of their electro-mechanical coupling and the unique ability to permanently alter their atomic-level dipole structure (i.e., their polarization) and to induce large-strain actuation through applied electric fields. Although the underlying multiscale coupling mechanisms have been investigated by modeling strategies reaching from the atomic level across the polycrystalline mesoscale to the macroscopic device level, most prior work has neglected the important influence of temperature on the ferroelectric behavior. Here, we present a phase-field (diffuse-interface) constitutive model for ferroelectric ceramics, which is extended to account for the effects of finite temperature by considering thermal lattice vibrations based on statistical mechanics and by modifying the underlying Landau-Devonshire potential to depend on temperature. Results indicate that the chosen interpolation of the Landau energy coefficients is a suitable approach for predicting the temperature-dependent spontaneous polarization accurately over a broad temperature range. Lowering the energy barrier at finite temperature by the aforementioned methods also leads to better agreement with measurements of the bipolar hysteresis. Based on a numerical implementation via FFT spectral homogenization, we present simulation results of single- and polycrystals, which highlight the effect of temperature on the ferroelectric switching kinetics. We observe that thermal fluctuations (at the phase-field level realized by a thermalized stochastic noise term in the Allen-Cahn evolution equation) promote the nucleation of needle-like domains in regions of high heterogeneity or stress concentration such as grain boundaries. This, in turn, leads to a faster polarization reversal at low electric fields and a simulated domain pattern evolution comparable to experimental observations, stemming from the competition between nucleation and growth of domains. We discuss the development, implementation, validation, and application of the temperature-dependent phase-field framework for ferroelectric ceramics with a focus on tetragonal lead zirconate titanate (PZT), which we demonstrate to admit reasonable model predictions and comparison with experiments.

Recyclable and Self-Repairable Fluid-Driven Liquid Crystal Elastomer Actuator.

Liquid crystal elastomer (LCE) is a newly emerging soft actuating material that has been extensively explored for building novel soft robots and diverse active devices, thanks to its large actuation stress and strain, high work density, and versatile actuation modes. However, there have also been several widely recognized limitations of LCE-based actuators for practical applications, including slow response and narrow range of operation temperature. Herein, we develop fluid-driven disulfide LCE actuators through facile laminate manufacturing enabled by a dynamic bond exchange reaction. Because of the merits of the active heating/cooling mechanism of the fluidic structure, this newly developed disulfide LCE actuator can generate large cyclic actuation at a frequency around 1 Hz and can operate in a wide range of temperatures. The unique combination of the fluidic structure design and the dynamic covalent bonds in the elastomer has also enabled the full recyclability and self-repairability of the actuator. Using the newly developed actuator as building block, we further constructed soft robotic systems that can realize manipulating and programmable movement. The design principle demonstrated in the current work opens a promising avenue for exploring more novel applications of LCE-based soft actuators.

Nonlinear dynamics of a conical dielectric elastomer oscillator with switchable mono to bi-stability

Dielectric elastomer actuators (DEAs) are an emerging type of soft actuator that show many advantages including large actuation strains, high energy density and high theoretical efficiency. Due to the inherent elasticity, such actuators can also be used as soft oscillators and, when at resonance, the dielectric elastomer oscillators (DEOs) can exhibit a peak oscillation amplitude and power output with an improved energy efficiency in comparison to non-resonant behaviour. However, most existing DEOs have a fixed pre-defined morphology and demonstrate a single stable equilibrium, which limits their versatility. In this work, a conical DEO is proposed which may exhibit either monostability (i.e. one stable equilibrium point) or bistability (two equilibria). The system demonstrates a transition between two regimes using a voltage control. Such a feature allows the DEO system to have multiple oscillation modes with different equilibrium points and the transition between equilibria is controlled by an effective control strategy proposed in this work. A mathematical model based on the Euler-Lagrange method is developed to investigate the stability of this system and its complex nonlinear dynamic response in unforced and parametrically forced cases. This design has potential in more advanced and versatile DEO applications such as active vibrational controllers/ shakers, active morphing structures, smart energy harvesting and highly programmable robotic locomotion.

Electrostatic field induced coupling actuation mechanism for dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) exhibit many fascinating advantages, including high compliance, large actuation strain, fast response, high energy density and efficiency. These muscle-like properties make them suitable for robotic and mechatronic applications especially the actuation of soft robots. However, such actuators have experienced high rates of failure when operating at high voltage, which impedes their practical application to some extent. Here, we report an enhanced and reliable actuation mechanism which couples the electrostatic induction actuation and the electrostatic attraction or zipping mechanisms for DEAs. A circular DEA (E-DEA), a zipping DEA (EZ-DEA) and a bending DEA (E-DEMES) are redesigned and fabricated using this mechanism, which both survive the pull-in electromechanical instability and demonstrate higher operational voltages, larger attainable deformations, and longer actuation lifetime. The E-DEA achieves a large area strain up to 174% and can sustain 20-kV or even higher voltage, its equivalent dielectric strength is larger than 877 MV/m. The EZ-DEA achieves a larger out-of-plane deformation up to 8 mm compared to the traditional zipping DEAs. The E-DEMES is promoted 5 times in response speed and maximally about 87% in the transient blocking force. The improved E-DEMES also exhibits a great potential in faster actuation, larger actuation stroke and force for soft robots.

Silica-decorated reduced graphene oxide (SiO2@rGO) as hybrid fillers for enhanced dielectric and actuation behavior of polydimethylsiloxane composites

A novel high performance dielectric elastomer actuator with a high dielectric constant, low dielectric loss, high dielectric breakdown strength and large actuated strain was prepared through introducing hybrid fillers of chemically reduced graphene oxide (rGO) covered by a SiO2 shell (SiO2@rGO). Hybrid particles were synthesized with two different shell coverage using tetraethylorthosilicate (TEOS) and characterized with different methods to identify the surface chemistry, inter-layer spacing, and thickness of the platelets as SiO2 shells were introduced and reduction process was applied. Composites of polydimethylsiloxane (PDMS) containing different amounts of rGO and SiO2@rGO particles were prepared by the solution mixing method. The results showed that composites with hybrid fillers have higher dielectric efficiency and dielectric strength than rGO/PDMS composites. However, higher actuated strains than the pure PDMS was obtained only with hybrid particles with dense coverage. Maximum actuated strain increased from 4.63% for the pure PDMS to 12.83% and 9.12% for composites with 1 wt% and 2 wt% of SiO2@rGO with denser SiO2 shell, respectively. Results suggest the positive effect of hybrid graphenic structures with high shell coverage and certain weight fraction toward preparing high performance dielectric elastomer actuator composites.

Silicone dielectric elastomer with improved actuated strain at low electric field and high self-healing efficiency by constructing supramolecular network

Dielectric elastomers (DE) suffer from cracks and breaks during repeated cycles, thus a DE material with both high actuated strain (SA) and self-healing ability is highly desired. Herein, for the first time we report a self-healable silicone DE (SiR-SN) with large SA under low electric field by constructing supramolecular network assembled by hydrogen bonds and ionic bonds from two components involving carboxyl modified polymethylvinylsiloxane (PMS-g-COOH) and amino terminated polydimethylsiloxane (PDMS-NH2). The results show that as PMS-g-COOH content increases, the dielectric constant (ε′) of SiR-SN increases owing to an increase in dipole content whereas the elastic modulus (Y) decreases because of a decrease in crosslinking density, leading to the significantly increased SA at a given electric field. SiR-SN with 0.2/1 of the mass ratio of PMS-g-COOH to PDMS-NH2 shows much higher ε′ and SA than the commercial silicone DE. Interestingly, self-healing of SiR-SN at 80 °C leads to the re-formation of hydrogen bonds, and thus the recovery of network structure, whereas self-healing at 100 °C can lead to the conversion from hydrogen bonds into ionic ones, and thus the change of network structure. As a result, a self-healing efficiency of 115% in tensile strength and almost 100% in SA at a given electric field can be achieved after self-healing at 80 °C for 5 h, whereas an increase in Y and higher breakdown strength can be achieved after self-healing at 100 °C, which may sustain the repeated stress and prolong the service life. It is promising that this new self-healable silicone dielectric elastomer with large actuated strain under low electric field could be used especially in biological and medical fields.

End-block-curing ABA triblock copolymer towards dielectric elastomers with both high electro-mechanical performance and excellent mechanical properties

Dielectric elastomers (DEs) featured with fast reversible and large deformation excited by an external electric field show promising applications as leading artificial muscle materials in many emerging technologies. It remains a great challenge to synthesize a DE simultaneously with high electromechanical performance (large actuation strain, high energy density, free of pre-stretch and bracing) and excellent mechanical properties (low Tg, low viscous loss, low modulus, and high mechanical strength). Herein, we propose and demonstrate that end-block-curing ABA triblock copolymer elastomer with well-designed structures is a new strategy to address the challenge by taking advantage of the microphase separation nature of the block copolymer. Particularly, poly(styrene-b-butyl acrylate-b-styrene) (SBAS) with post-curable end blocks, i.e. polystyrene (PS) blocks, are designed and synthesized. Different levels of cyclohex-3-enylmethyl acrylate (CEA) that contains two asymmetric vinyl groups with very different reactivity are incorporated into the end blocks for end-block curing while the molecular weight for each block is fixed. It is found that curing the end blocks enhances the strain-hardening effect during stretch and thus significantly increases the electro-mechanical performance. Without pre-stretch, the DE actuator of the end block cured SBAS exhibits a maximum actuation strain up to 72%, dielectric breakdown strength up to 154 kV mm−1, and energy density up to 270 kJ/m3, which are three, four, and almost fourty times that of the SBAS free of CEA, respectively. The maximum energy density is over 30 times that of mammalina skeletal muscle while the maximum actuation strain is 3.4 times. Interestingly, the improvement in eletromechanical performance is fulfilled with little increase in glass transition temperature (Tg), mild increase in modulus, and excellent tensile strength due to the microphase structures. The end-block-cured SBAS retains a low Tg of the rubbery phase around −35 °C, relatively low viscous loss of 0.2 at room temperature, a modulus lower than 1 MPa, and tensile strength around 3 MPa, which are attractive in terms of DE device applications.

Dielectric elastomer materials for large-strain actuation and energy harvesting: a comparison between styrenic rubber, natural rubber and acrylic elastomer

This paper compares the performance of commercially available membranes made of styrenic rubber, natural rubber and acrylic elastomer for dielectric elastomer transducers (DETs) operating in the large strain regime. Following a detailed description of the adopted experimental set-up and procedures, the results of a comprehensive electro-mechanical characterization of the three materials are reported to highlight the following dependencies: dielectric strength versus stretch, electrical conductivity versus electric field, dielectric constant versus stretch, stress versus stretch and strain rate. This includes the fitting of the experimental data with constitutive equations which provide material property values that can be used for model-based analysis, design and control of dielectric elastomer actuators and generators operating at large levels of strain amplitudes (like, for instance, transducers featuring actuation and generator strains over 50%) or in the presence of large pre-strains (over 50%). Performance metrics relying on the identified constitutive parameters are introduced in order to discuss the specific pros and cons of the considered elastomers for the development of practical DETs.

Reversible and High-Temperature-Stabilized Strain in (Pb,La)(Zr,Sn,Ti)O3 Antiferroelectric Ceramics.

Antiferroelectric (AFE) materials have a tremendous advantage as smart materials and large-strain actuators due to their unique reversible characteristic electric-field-induced strain (electrostrain) responses in comparison to piezoelectric effect and electrostriction. A key limitation to today's AFE actuators, however, is the poor temperature stability of electrostrain. In this work, a large reversible strain of 0.4% and an excellent thermal stability with a variation within ±5.5% from 20 to 190 °C were achieved for (Pb0.97La0.02)(Zr0.85Sn0.08Ti0.07)O3 (PLZST) AFE ceramics. A room-temperature electrostrain of 0.71% was obtained in virgin PLZST ceramics. It is intriguing to observe inconsistent strain curves between the first and further measured cycles, implying an incomplete reversible field-induced AFE-ferroelectric phase transition. A sharp electrostrain response in milliseconds was realized in the as-prepared PLZST ceramics. In addition, a phenomenological explanation was proposed to explain the extraordinary phenomena. Our results may shed light on the origin of the superior strain behaviors in AFE materials from the view of microscopic structure and macroscopic properties, and probably improve the understanding of the AFE phase transition.

Nanoscale silicon-based actuators with extremely large actuation strain and extremely low driving voltage

The coupling between lithiation reaction of silicon and the considerable volume change has been widely recognized as an adverse effect which hinders the practical application of silicon-based lithium-ion batteries. Here we theoretically demonstrate a novel class of nanoscale electrochemically-driven silicon actuators, in virtue of the “unfavorable” gigantic volume expansion engendered by lithiation. Two representative design prototypes are reported, namely, a nano-sized flat-film silicon actuator and a nanowire silicon actuator. Our thermodynamic analysis establishes the operation condition of the actuators by identifying the electrochemical driving force and mechanical resistance due to lithiation-induced stress. We show that the nano-actuator exhibits an extremely low driving voltage about 1 V and an extremely high strain of actuation up to 300%, which goes far beyond the features of most common actuator materials. Given a mechanical load, the flat-film silicon actuator features a constant actuation strain and the nanowire actuator can provide tunable actuation strain. The results from the study offer quantitative guidance to the design of the novel silicon-based nano-actuators.

On the nonlinear dynamics of a circular dielectric elastomer oscillator

Dielectric elastomer actuators (DEAs) are an emerging soft actuation technology that have advantages in large actuation strain, inherent compliance and low cost. Resonant actuation of DEAs has been shown to increase the actuation stroke and power output and has been utilized in applications such as robotic locomotion and loudspeakers. In this work, we present a planar circular dielectric elastomer oscillator (DEO) that can exert out-of-plane deformation as large as 80% of its own diameter at resonance. By experiments and model simulation we show that this simple DEO design exhibits complex nonlinear responses, including multiple solutions, sub- and super-harmonics, and exhibits multiple resonant peaks. The DEO performance is characterized against membrane pre-stretch and the weight of the attached mass and its actuation signals (DC and AC amplitudes). The resonant frequency and the amplitude are found to be easily tuned by varying these parameters. Such large stroke output and highly tunable resonance make this DEO a promising candidate for applications such as active vibration absorption, embedded loudspeakers, soft pumps and robotic locomotion.

Long Liquid Crystal Elastomer Fibers with Large Reversible Actuation Strains for Smart Textiles and Artificial Muscles.

A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a two-stage thermal-photo curingreaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel, then mechanically stretched, and photocured to achieve liquid crystal chain alignment for stress-free reversible activation. Upon optimizing the ink viscosity and DIW printing parameters, long fibers (up to 1.5 m long from the laboratory) were obtained. The resulting fiber had a modulus of 2 MPa, 51% actuation strain, and a failure strain of well over 100%. The potential of these fibers for applications was demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and biosensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.

Electrical and Mechanical Self‐Healing in High‐Performance Dielectric Elastomer Actuator Materials

AbstractDielectric elastomers are of interest for actuator applications due to their large actuation strain, high bandwidth, high energy density, and their flexible nature. If future dielectric elastomers are to be used reliably in applications that include soft robotics, medical devices, artificial muscles, and electronic skins, there is a need to design devices that are tolerant to electrical and mechanical damage. In this paper, the first report of self‐healing of both electrical breakdown and mechanical damage in dielectric actuators using a thermoplastic methyl thioglycolate–modified styrene–butadiene–styrene (MGSBS) elastomer is provided. The self‐healing functions are examined from the material to device level by detailed examination of the healing process, and characterization of electrical properties and actuator response before and after healing. It is demonstrated that after dielectric breakdown, the initial dielectric strength can be recovered by up to 67%, and after mechanical damage, a 39% recovery can be achieved with no degradation of the strain–voltage response of the actuators. The elastomer can also heal a combination of mechanical and electrical failures. This work provides a route to create robust and damage tolerant dielectric elastomers for soft robotic and other applications related to actuator and energy‐harvesting systems.